Many prosthetic medical devices are implanted into load-bearing joints such as knees, hips, etc. As such, these prosthetic devices must be very strong and possess a high degree of wear resistance. Presently, the prosthetic medical device industry has utilized various metals and polymers and combinations thereof to fabricate prosthetic devices. Unfortunately, both metals and polymers have drawbacks. For example, metals such as stainless steel, tungsten and titanium, and alloys thereof, may succumb to the corrosive environment of the body and eventually begin to wear. Such wear may result in fine metallic particles being scraped away from the contact surface of the device and into surrounding tissue and bone which may potentially cause pathogenic problems. Polymers, such as polyethylene, polypropylene and nylons may also exhibit wear and may consequently produce particles which diffuse into tissue and bone. Both metallic and polymeric particles shed from these prosthetic medical devices are of concern because they may be inherently reactive with the tissue and bone they contact, thus possibly causing tissue degradation or necrosis.
Various methods have been devised attempting to reduce the wear rate of the load bearing prosthetic medical devices. For polymers, a common practice within the prosthetic medical device industry is to use cross-linked polymers and resins to form the medical device. Polymers are commonly cross-linked by chemical catalysis or irradiation exposure. Most cross-linking methodologies do result in greater wear resistance. However, indiscriminate or uncontrolled cross-linking may result in the formation of a weakened polymeric matrix, not capable of withstanding the enormous pressures placed on the devices in the patient resulting in degradative wear as described above.
Another difficulty conventionally encountered in the manufacturing process of prosthetic medical devices is that they cannot be formed by inexpensive injection molding techniques. Instead, these medical devices must be formed by extrusion, for example, which requires further machining into the finished article. Injection molding, on-the-other-hand, allows for the final article to be formed in virtually one step.
Therefore, a need exists within the prosthetic medical device industry to fabricate an improved polymeric prosthetic device possessing sufficient strength to withstand the stress and pressure imposed on it, yet resist wear so that foreign particles liberated from the prosthetic device do not cause health problems to the patient. There also exists a need to fabricate the devices inexpensively by injection molding. The present invention provides compositions, as well as methods of improving the wear resistance of prosthetic medical devices, by selectively cross-linking a polymeric resin using a controlled cross-linking process providing optimum strength and wear resistance, thus diminishing or eliminating the frequency by which foreign particles are liberated from the implanted prosthesis, thereby reducing the risk of compromising the patient's health. The present invention also provides compositions and methods of injection molding prosthetic medical devices thus rendering a less expensive, and more facile prosthetic medical device fabrication process.